The development of solid formulations that disintegrate quickly in the mouth without requiring water are very interesting due to the advantages that these pharmaceutical formulations provide for patients who have difficulty in swallowing, such as old people, infants, patients with mental problems and non-cooperative patients, as well as the population in general; since it makes it possible for the drug to be administered without the need for water. Moreover, since the formulation disintegrates inside the mouth, the drug may be absorbed in the oral, pharyngeal and gastrointestinal regions. The pre-gastric drug absorption avoids hepatic first-pass metabolism.
In the European Pharmacopoeia (Ph.Eur.) 5th edition, Supplement 5.2, published in June 2004, orodispersible tablets are defined as non-coated tablets for placing in the mouth which disintegrate quickly before they are swallowed. It also establishes 3 minutes as the time under which they must disintegrate in the disintegration test for tablets and capsules, according to the Ph. Eur. 2.9.1. method.
Several technologies are available to produce commercial oral fast-dissolving systems. The technologies are usually grouped according to the method employed in the preparation: freeze drying (Zydis®, Quicksoly° and Lyoc®), molding (FlashDose® and compaction technologies.
Freeze drying (lyophilization) is a process in which a solvent is removed from a drug solution or a drug suspension containing structure-forming excipients. The tablets are characterized by a highly porous network; they quickly absorb water and dissolve, releasing the incorporated drug. The freeze drying process occurs at a low temperature, which eliminates the adverse thermal reactions that may affect drug stability. However, the freeze-drying process is very expensive and problematic when scaling up. In addition, the resulting dosage form is characterised by high friability, low stability at high temperatures and humidity levels, and by showing poor mechanical properties. Moreover, sometimes, special packaging is required.
The molding technology can be based in two different processes: the solvent method and the heat method. The solvent molding process involves preparation of a drug solution or suspension that contains a drug and the excipients and evaporating the solvent at ambient pressure and drying. In the heat molding method, the tablets are formed using a candy floss or shearform matrix, which is composed of saccharides or polysaccarides processed into amorphous floss by the simultaneous action of flash melting and centrifugal forces. The molding technology allows preparing high drug dose tablets and the resulting tablets present a rapid dissolution. However, because of their low mechanical strength, molded tablets are subject to erosion and breakage during the handling and opening of the blister pockets. Moreover, taste masking is an additional requirement with this technology.
The conventional process used to prepare fast-dissolving tablets has some advantages, such as being a well established technology, having low manufacturing cost and allowing easy technology transfer (e.g. easy to transfer to different producers). Many strategies for developing tablets with high porosity and suitable mechanical strength have been attempted, including: granulation (wet granulation, dry granulation and mold granulation) followed by compression; and direct compression.
In the granulation process, fast dissolving tablets are prepared by mixing the granules with a superdisintegrant and other appropriate excipients and compacting to obtain tablets capable of quickly disintegrating in the mouth with a limited amount of saliva. Formulation based on this technology is FlashTab® to Prographarm which is described in European patent EP 0 548 356.
On a related matter, EP1681048 describes an orally disintegrating tablet of olanzapine prepared with granulation as intermediate step and subsequent compression of the granules. A similar method is also disclosed in DE102005009241 and IP. Com. Journal, 2006, but they incorporate other active ingredients. EP1488811 refers to orally disintegrating tablets of pravastatin which are obtained by compression moulding of prepared granules. EP1674083 discloses fast disintegrating tablets prepared by first granulating a dispersion containing the ingredients in a spray-drying device, mixing the obtained granules with the active ingredient and magnesium stearate and finally, subjecting the mixture to a tabletting process.
Although tablets obtained through the granulation methodology demonstrate rapid dissolution, its rate is correlated to the hardness of the tablet and can be slower than freeze-dried tablets. Moreover, possible problems can be related to drug stability after granulation.
Direct compression represents the simplest and most cost-effective tablet-manufacturing technique from a technological point of view. Fast-dissolving tablets can be prepared by using suitable excipients with improved properties. Two known formulations based on this technology are Ziplets® to Eurand which are described in international patent application WO 99/44580 and DuraSoly® by Cima described in U.S. Pat. No. 6,024,981. Direct compression based technologies uses suitable excipients with improved properties, most notably superdisintegrants which accelerate the rate of disintegration and hence dissolution. Water soluble excipients and, sometimes, effervescent agents assist in the disintegration process. Addition of insoluble compounds which increase the efficiency of the superdisintegrant: the disintegration time decreases as the amount of hydrophilic insoluble compound increases.
Ziplets® technology is used to obtain taste masking and fast release of water-soluble or water insoluble drugs from microcapsules and granules. The resulting fast-dissolving tablets are obtained by direct compression of mixtures that contain at least one inorganic excipient that is insoluble in water, for example, calcium phosphate, one or more disintegrants, for example, crospovidone and optionally, water soluble excipients. However, the resulting compositions contain a high percentage of insoluble excipients which leave a high amount of residue in the mouth and jeopardise their palatability.
The DuraSolv® technology is designed to provide stronger tablets without packaging precautions. This technology is based on employment of conventional non-direct compression fillers (such as dextrose, mannitol, sorbitol, lactose and sucrose) in the form of fine particles that quickly dissolve without producing a gritty or sandy sensation in the mouth.
However, all the above processes for obtaining orodispersible tablets involve, to a greater or lesser extent, the following disadvantages:                A high content of insoluble excipients or microencapsulated active ingredients that give the formula a gritty feel after they have been disintegrated in the oral cavity and, consequently, problems with palatability.        Excessively long disintegration times in comparison with oral lyophilisates or wafers, which, in general, dissolve in less than 10 seconds.        Insufficient mechanical resistance to resist conventional packaging and transport operations.        
U.S. Pat. No. 6,610,266 describes the preparation of calcium metasilicates with low aspect ratio, and its use to prepare fast-disintegrating tablets by direct compression. However, large amounts of this excipient, about 40% by weight, are required for preparing the tablets, thus negatively affecting the size as well as the palatability of the tablet.
International application WO03/030868 also discloses the use of calcium silicate to prepare flashmelt oral dosage formulations. In this case, in addition to use even higher proportions of said excipient, the manufacturing process is more complex since it comprises a previous granulation of the ingredients.
Therefore, it is an object of this invention to provide orodispersible tablets which can be dissolved quickly in the oral cavity, which presents good palatability properties and that can be obtained by conventional processes, such as direct compression.